Smallpox Vaccine Data Show Small but Serious Risk of Infecting Others

From CDC National Prevention Information Network

October 16, 2002

A note from TheBody.com: Since this article was written, the HIV pandemic has changed, as has our understanding of HIV/AIDS and its treatment. As a result, parts of this article may be outdated. Please keep this in mind, and be sure to visit other parts of our site for more recent information!

According to a new report on millions of smallpox vaccinations given in the 1960s, there is a small but significant risk that newly vaccinated people can make others seriously ill by infecting them with vaccinia, the virus used in the vaccine.

The study sought to clarify issues surrounding smallpox vaccination as the United States, fearing a bioterror attack, prepares to vaccinate hundreds of thousands of soldiers and health care workers and considers whether to offer the vaccine to the pubic. Routine smallpox vaccination in the United States ended in 1972. Today a major concern is the possibility that newly vaccinated people might inadvertently infect others, particularly those at high risk. The report, "Contact Vaccinia -- Transmission of Vaccinia from Smallpox Vaccination," appears in today's Journal of the American Medical Association (2002;288;15).

The analysis of 11.8 million Americans vaccinated for the first time in 1963 and 1968 found that for every 100,000, vaccinia spread by close contact to two to six unvaccinated persons. Most unvaccinated people who caught the virus developed "accidental infections," sores that healed on their own, and most likely acted like secondhand vaccinations and gave immunity. But one or two became very ill with eczema vaccinatum, which affects some people with eczema or even a history of it and is occasionally fatal. Eczema vaccinatum can be treated with vaccinia immune globulin, but this is in very short supply.

Study author Dr. Donald A. Henderson, a government adviser on bioterrorism, noted that this transmission was "almost entirely kid to kid, mostly within a household." But he added, "It's a different population today, and so we're looking at uncertainties."

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Today's population is different because, for unknown reasons, eczema is two to three times more common than in the past. Also, more people have lowered immunity -- from HIV, other immune system diseases, or drugs used to treat cancer or prevent organ transplant rejection. People with immune system disorders are at risk, either through being vaccinated or through contact, for the potentially fatal condition progressive vaccinia, in which the sore at the vaccination site keeps growing and spreading, and systemic illness develops. There is no cure for progressive vaccinia, according to study author Dr. John M. Neff.

Neff said his greatest concern was the threat of progressive vaccinia in immunocompromised people, either those vaccinated who should not have been or the contacts of vaccinated people. The CDC estimates that 300,000 Americans have HIV and do not know it. In addition, more than 23,000 health care workers have AIDS.

The researchers said they knew of only one case in which an HIV-positive person was given the smallpox vaccine. The patient, a 19-year-old soldier, did not know he had HIV when he was vaccinated by the military in 1984. He developed progressive vaccinia and AIDS, and died.

A note from TheBody.com: Since this article was written, the HIV pandemic has changed, as has our understanding of HIV/AIDS and its treatment. As a result, parts of this article may be outdated. Please keep this in mind, and be sure to visit other parts of our site for more recent information!

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